A classifier is a computerized tool that classifies an input set of data according to a finite set of classes. One general form of classifier uses machine learning techniques to make classification decisions. Before using this type of classifier, it is necessary to train the classifier using preclassified data, referred to as labeled data. In general, the more labeled data that is available to train the classifier, the more accurate the classifier can be. However, labeled data is typically produced manually and is therefore expensive in terms of time and money. The expense of this type of data ultimately limits the practical application of classifiers.